1. Technical Field
The embodiments herein generally relate to wireless communication systems, and, more particularly, to mobile television (TV) multicast technology.
2. Description of the Related Art
The MediaFLO™ (Forward Link Only) mobile multimedia multicast system available from Qualcomm, Inc., California, USA, is an alternative mobile multicast technology for the efficient transmission of multimedia data streams to mobile devices using TV. One radio frequency (RF) channel is shared among many TV channels (TV programs). These TV channels are multiplexed either in the time domain or in the frequency domain. When the TV channels are multiplexed in the time domain, each channel is given full access to the entire RF channel bandwidth for a short period of time (burst duration). After that burst is transmitted, bursts for other channels occupy the RF channel and so on. This multiplexing process is called time division multiplexing (TDM)
Multimedia streams can comprise various channels coming from different sources. The smallest channel transmission unit is called a Multicast Logical Channel (MLC). Each MLC is composed of three streams plus parity bits for error detection as indicated in FIG. 1, then each MLC is divided into four bursts, such that one burst per MLC is transmitted per frame, and every four frames are packed together in addition to overhead information to form a Super-Frame of duration one second. These streams are multiplexed/demultiplexed to/from a single MLC in a layer called the stream layer.
Streams 1 and 2 carry audio and video information of the channel requested by the user, while stream 0 carries control information used in the decryption of these audio and video information at the receiver, a number of MLCs are multiplexed together in the time domain to form a frame, and then four frames are packed together in addition to overhead information to form a super-frame having a duration of one second.
A super-frame is divided into four portions as shown in FIG. 2: TDM pilots, Overhead Information Symbols (OIS), Data, and Pilot Positioning Channel (PPC). TDM1 is the first symbol of a super-frame and marks its beginning. In addition to being used for frame synchronization, TDM1 can also be used for initial time and frequency synchronization, so the receiver can immediately start decoding the information carried by the OIS. The OIS contain the time-frequency allocation for each MLC in the current super-frame. With respect to data, when an MLC is transmitted in a super-frame the payload is divided into four equal bursts, with each burst transmitted in a unique frame. The PPC can be used for transmitter identification and/or position location at the receiver. Super-frames are transmitted in sequence to be received at the service user. To ensure correct reception there must be synchronization between the transmitter and the receiver, and then several steps are performed to get the data streams ready for display.
A receiver which is watching only one channel (for example, CH2) needs only to be active (ON) during the periods of CH2 bursts. In order to conserve battery consumption, such receiver will shut off its circuits when CH2 bursts are not occupying the RF channel. It thus becomes in a SLEEP mode. This shows that TDM of channels can help reduce power consumption of a receiver watching a single channel. When the user switches to another channel, the receiver executes these steps: First, the receiver looks for the TDM1 (a symbol which indicates the beginning of a super-frame) as shown in FIG. 3. Then, it looks for the OIS (five symbols contain the information about the specific location of the MLC) in the current Super-Frame that belongs to the Channel required by the user. Second, as the receiver reaches the MLC in any of the four frames, it starts looking for stream 0 to extract several messages (Entitlement Control Messages (ECM) and Encryption Information Messages (EIM)) from it. Third, the receiver uses these messages, in addition to a message related to each user (Entitlement Management Message (EMM)) defined by the service provider, to be able to extract the code word (CW), which is the key used in a descrambling process. Finally, the receiver uses that CW to descramble the audio and video information contained in streams 1 and 2. This scenario is repeated every super-frame to guarantee efficient reception of data. However, this causes a problem; an example is shown in FIG. 4, if the user switches from CH2 to CH3.
The worst case occurs when the user issues a command to switch to CH3 right after the burst of CH3 ends. In this case, the receiver has to wait until the next burst that belongs to CH3 appears on the RF channel (in the next super-frame to get its OIS data) as in FIG. 4. This causes the user to wait for a given period of time denoted as the channel switching delay. This means that the delay introduced when the user turns on the channel is very large and also during switching from one channel to another; i.e., long channel switching delay. Accordingly, there remains a need to enhance the stream layer transmission for the MediaFLO™ mobile multimedia multicast system.